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Formaldehyde gas sensor based on hexagonal-molybdenum trioxide

Author

Dharmanto, Theodora Karin

Date of Issue

2017

School

School of Materials Science and Engineering

Abstract

Formaldehyde is a volatile organic compound that is widely used in industry and household materials. Formaldehyde is continuously outgassed because of its high vapor pressure, resulting a prolonged low-level exposure which causes adverse health effects.
In order to detect the presence of formaldehyde, various metal oxides based gas sensors have been fabricated and developed. Hexagonal MoO3 (h-MoO3) is an important metastable transition semiconducting metal oxide with unique functional properties due to its hexagonal framework. It has shown good response towards several VOCs, such as acetone and ethanol. The application of organic/MoO3 hybrids for formaldehyde gas sensors has been extensively studied. However, the application of pure h-MoO3 for formaldehyde gas sensors has not been studied yet. Therefore, pure hexagonal molybdenum trioxide (h-MoO3) will be the focus in this study.
Investigation on the suitable type and concentration of mineralizer (Na2SO4, NaNO3, and C24H39NaO5.xH2O) to fabricate h-MoO3 by hydrothermal method was conducted. X-Ray diffraction (XRD) was done to investigate the crystal structure of the resulting samples. Scanning electron microscope (SEM) was utilized to observe the physical micro-structure of the samples. Thermogravimetric analysis (TGA) was performed to analyse the thermal stability and chemical composition of the samples. XRD pattern showed that the sample containing 250 mg of Na2SO4 in the peroxomolybdate precursor (H250) had h-MoO3 phase. The rest contained a mixture of h-MoO3/α-MoO3 or pure α-MoO3 phase.
The h-MoO3 samples was then drop casted as the sensing layer onto an alumina substrate with previously patterned interdigitated gold electrode. A study on gas sensing response showed that the H250 gas sensor exhibited best response towards formaldehyde at room temperature, due to its pure metastable structure which provides less energy for surface reaction with formaldehyde.
The study was extended using screen printing method to achieve a more consistent chemical performance as compare to drop casting method. Ultra-sonication was conducted in three sonication period (2, 4, and 6 hours) to achieve smaller particle size. SEM shows H250 sonicated for 6 hours (S6) had the smallest particle size. Then, screen-printed paste, consisted of α-terpineol, ethyl cellulose powder, and H250 S6 sample powder, was screen printed onto alumina substrate with pre-patterned interdigitated gold electrode. It also showed stable response towards formaldehyde gas vapour.
In the last section of the study, the relation of operating temperature with the gas response of H250 S6 gas sensor were investigated by using a commercial heater. There was no significant difference in the responses towards formaldehyde at various temperatures (25 ̊C, 85 ̊C, 150 ̊C) for the gas sensing in this study.